Oscillation generator



Feb. 14, 1939. M. s. MEAD, JR

OSCILLATION GENERATOR Filed Oct. 17, 1935 2 Sheets-Sheet l W a a 411WJHQZWMW 4% 1 4 mam $0M? in m H\ Z J) 3 .n M, w

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Invehtor:

1m y w m e P dm 8. 5M m a W H 1785- 1939- M. s. MEAD, JR

05C ILLATION GENERATOR Filed Oct. 1'7, 1935 2 Sheets-Sheet 2 InventorMilton S. Mead, Jr;

b is Attorney Patented Feb. 14, 1939 2,147,492

UNITED STATES PATENT OFFICE OSCILLATION GENERATOR Milton S. Mead, Jr.,Schenectady, N. Y., assignor to General Electric Company, a corporationof New York Application October 17, 1935, Serial No. 45,379

14 Claims. (01. 250-36) My invention relates to oscillation generators,in the amplifier anode circuit substantially conand more particularly tooscillation generators stant. employing an electron discharge controltube and In oscillators of the above general arrangeincluding afrequency determining system of the ment it has been found to bedesirable to emmechanically vibratory type. ploy a screen grid tube asthe amplifier control 5 In certain applications it is necessary toemelement. The presence of the extra grid preploy oscillators having anoutput frequency which vents undesired interelectrode coupling betweenremains substantially constantover long periods the control grid andanode of the tube. In this of time and under all operating conditions.One type of tube, the magnitude of positive potential such applicationis that of clock supervisory syssupplied to the screen grid determinesin large 10 terns wherein an oscillator is used to control the part theamplitude of the current flowing in frequency of the current supplied todrive the the anode circuit of the tube. Since the desirclocks connectedin the system. Oscillators ability of maintaining this currentsubstantially adapted for this purpose usually include a meconstant,thereby to maintain the magnitude of chanically vibratory element as,for example, a current traversing the drive coil constant has 15 tuningfork, having drive and pick-up coils assobeen previously pointed out, itwill be seen to ciated therewith; which coils are respectively bedesirable to maintain the magnitude of screen coupled to the output andinput circuits of an grid potential substantially constant. electrondischarge amplifier and are so arranged An additional object of myinvention is to prothat vibration of the element controls the amvide, inan oscillation generator of the charac- 20 plifier to produce sustainedoscillations in the ter stated, means comprising a circuit connectedoutput circuit thereof. In this type of oscillato supply a biaspotential to the control grid of tor a portion of the energy developedin the amthe amplifier which varies in accordance with the plifieroutput circuit is supplied to the drive coil magnitude of current in theamplifier anode cirfor the vibratory element to maintain the elecuit formaintaining substantially constant the 25 ment in sustained vibration.magnitude of current supplied to the drive coils It is an object of myinvention to provide an of the vibration element, thereby to maintainoscillation generator of the above-described type the amplitude ofvibration of the element subwhich is of improved construction, which isquick stantially constant.

starting and which is capable of producing oscil- A still further objectof my invention is to 0 lations of extremely constant frequency underprovide in an electron discharge oscillator havall operating conditionsand for long periods of ing a mechanically vibratory frequencydetertime. mining element and including an electron dis- It has beenfound in oscillation generators of charge tube having an auxiliaryscreen grid,

the above type that the frequency of the genermeans for maintainingsubstantially constant the 5 ated oscillations varies in accordance withthe value of the potential impressed on the auxiliary amplitude ofvibration of the vibratory element. grid.

t has further been found that the amplitude of It is extremely difficultto maintain the sensivibration of the vibratory element varies inactivity of the coupling amplifier between the drive cordance with themagnitude of current supplied and pickup coils suificiently stable toeliminate 40 to t e d ve Co o the put circuit of the all variations inthe amplitude of vibration of electron discha e ube amplifier. It istherethe mechanically vibratory element. In certain for ne s y t pr v ai a m an fo types of oscillators known to the art such sensimaintainingsubstantially constant the intensity tivity variations are accompaniedby a dispropor- 5 of the alternating current supplied to the drivetionately greater variation in the amplitude of 5 @0 18 if rigidfrequency regulation s to be vibration of the fork tines. By providing acirtained. cuit for supplying a biasing potential to the con- Inaccordance with my invention, the intensity trol grid which varies inaccordance with the of current supplied to the drive coils of thevibraintensity of current in the output circuit, an

0 tory element is maintained substantially constant arrangement isobtained wherein unavoidable by the provision of a circuit arranged tosupply a variations in the sensitivity of the amplifier are biasingpotential to the amplifier tube control grid accompanied only by aproportionate change in which varies in accordance with the magnitudethe amplitude of vibration of the fork tines. In of current in theamplifier output circuit, thereother words, the relation between changesin the ,5 by to maintain the magnitude of current flowing amplitude ofvibration and changes in amplifier sensitivity is linear which, ofcourse, represents the smallest obtainable frequency variation for agiven amplifier sensitivity change.

Accordingly, a still further object of my invention is to provide anoscillation generator of the type including a frequency determiningelement comprising a vibratory element in which a substantially linearrelation exists between variations in the sensitivity of the amplifierand variations in the amplitude of vibration of the element.

One conventional form of tuning fork oscillator includes means forproducing a unidirectional flux which traverses the field structure ofthe pick-up coil; which fiux' is varied in intensity during vibration ofthe fork tines to generate a control potential between the terminals ofthe pick-up coil. The unidirectional flux may thread i the tines of thefork, whereby the stiffness of the fork is altered and the naturalperiod of vibration of the fork tines is changed. In the embodiment ofmy invention to be described hereinafter the above-mentioned meanscomprises a permanent magnet which is magnetically connected to thefield structure upon which the drive and pick-up coils are wound. It isdesirable to employ an arrangement wherein the intensity of theunidirectional fiux may be easily varied thereby to change the stiffnessof the fork tines and to produce a change in the frequency of vibrationthereof. It is further desirable to provide such an arrangement in whichthe intensity of the unidirectional flux is maintained substantiallyconstant following an adjustment thereof and during operation of theoscillator. In accordance with my invention, the intensity of theunidirectional flux may be varied at will by bucking or boosting themagnetization of the permanent magnet which produces this fiux. Uniformintensity is obtained by providing conductors closed circuited on theunidirectional flux path in such a manner that instantaneous changes inthe fiux intensity are opposed.

It is a further object of my invention to provide in combination with atuning fork controlled oscillator having the previously describedcharacteristics, improved means for varying the intensity of theunidirectional flux threading. the fork tines thereby to vary thestiffness of the fork and the rate of vibration of the fork.

It is a still further object of my invention to provide means associatedwith the field structure of the frequency determining element foropposing instantaneous changes in the intensity of the unidirectionalflux traversing the magnetic circuit provided for this flux.

It has been found that magnetic coupling between the drive and pick-upcoils produces voltages in the two coils which cause fundamental andharmonic forces to be set up which influence the vibration of the forktines. These forces cause the tines to vibrate in such a manner thatharmonics are developed in the control potential between the terminalsof the pick-up coil. In accordance with my invention, the above-notedundesired features are obviated by loosely coupling the drive andpick-up coils and by providing a transformer having windings connectedrespectively to the drive and pick-up coil circuits to produce voltageswhich neutralize the voltages developed by the coupling between the twocoils. In this manner undesired forces acting on the fork tines areeliminated and the tines are permitted to vibrate at a frequencydetermined by the natural period of the fork.

An additional object of my invention is to provide an oscillatorcontrolled by a vibratory system which includes loosely coupled driveand pick-up coils magnetically associated with a vi-' bratory element,and means connected in circuit with the coils for neutralizingpotentials generated in either coil due to the coupling between thecoils, whereby the voltage generated in the pick-up coil is producedsolely by vibration of the vibratory element to vary the unidirectionalfiux linking the turns of the pick-up coil.

The novel features which I believe to be characteristic of my inventionare set forth with particularity in the appended claims. My inventionitself, however, both as to its organization and the method ofoperation, together with further objects and advantages thereof, willbest be understood by reference to the following specification,

taken in connection with the accompanying drawings, in which Fig. 1illustrates an improved circuit arrangement for an oscillator having myinvention embodied therein; Fig. 2 is an enlarged view of an element ofthe apparatus shown in Fig. 1 illustrating the flux paths in the tuningfork field structure; Fig. 3 illustrates certain of the operatingcharacteristics of my improved oscillator; Fig. 4 illustrates thecorresponding characteristics of oscillators typical of the prior art;Figs. 5 and 6 are side views partially in section showing the details ofmy improved fork field structure assembly; and Fig. 7 is a view inelevation showing the details of the tuning fork control mechanism.

Referring to Figl of the drawings, I have shown my improved oscillationgenerator as comprising an amplifier including a tube l which iscontrolled by a vibratory element 2 to generate electrical oscillationshaving a frequency determined by the frequency of vibration of theelement 2. The vibratory element 2 is shown as a tuning fork, althoughit will be understood that other forms of mechanically vibratoryelements may be employed without modification of the circuitillustrated. The output from the tube l is impressed on the inputcircuit of a second amplifier tube 3 which in turn has its outputcircuit coupled to a utilization circuit (not shown) by the leadsindicated at 4. V

The electron discharge tube l is shown as being of the single diodepentode type and includes a cathode 5, an anode 6, a'control grid 7, ascreen grid 8, a cathode heater 9, a diode electrode ID, and asuppressor grid ll the latter grid being directly connected to thecathode 5 and functioning to prevent secondary electron emission withinthe tube. The electron discharge tube 3 is similar in all respects tothe tube l with the exception that the single diode electrode isomitted.

The input circuit to the amplifier tube l includes the secondary 52 of acoupling transformer l3 having its primary Ml connected to a pick-upcoil I5 wound on the field structure of the tuning fork 2. The inputcircuit is connected between the control grid 1 and the cathode 5 of thedischarge device I through an alternating current by-pass condenser iii.In order to adjust the phase angle between the current supplied to thetuning fork drive coil and the voltage developed across the terminals ofthe pick-up coil Hi, thereby to adjust the frequency of vibration of thefork tines, an adjustable resistance I9 is connected in the circuitconnecting the pick-up coil l5 and with the primary winding 55-. Onewinding 20 of a transformer 2% is also included in this circuit for apurpose tobe described hereinafter.

The output circuit of the amplifier includes'the primary winding 22 of acoupling device 23 having its secondary winding 24 connected to excite afork drive coil 25 through a second winding 26 of the transformer 2|.The winding 22 is connected between the anode 6 and the cathode 5 of thedevice I in series with a source of high voltage direct currentpotential 2! and a resistance 28. A condenser 29 is connected in shuntwith the winding 22 and functions to tune the inductance of the windingto form an oscillatory circuit 38 having a resonant frequency equal tothe normal operating frequency which the apparatus is designed toproduce.

The potential developed across the terminals of the oscillatory circuit30 is impressed upon a resistance 3| connected to the oscillatorycircuit by a pair of alternating current by-pass condensers 32 and 33,included in the input circuit of the tube 3. The resistance 3| isconnected between the control grid and cathode of the discharge device 3through a by-pass condenser 34. A resistance 35 connected at one end tothe negative side of the voltage source 2! serves to bias the controlgrid of the tube 3 negative with respect to the cathode thereof.

The output circuit of the discharge device 3 includes the leads 4connected respectively to the anode and cathode of the device 3 andshunted by the tuning condenser 36. A source of anode voltage comprisingthe potential drop across a pair of series-connected resistances 3'! and38 is also in cluded in the output circuit between the cathode of thetube 3 and one terminal of the condenser 36.

A cathode heater circuit for the tubes l and 3 is provided whichincludes the respective heaters of the two tubes connected in serieswith each other and in series with voltage dropping resistors 31, 38,and 39 across the high voltage source 2?. A positive potential issupplied to the screen grid of the tube 3 through a circuit whichincludes the voltage-dropping resistor 38. This resistor serves tomaintain the screen grid at a positive potential slightly lower than thepositive potential of the anode of the device 3. In similar manner, abiasing potential is impressed on the screen grid of the tube throughthe circuit lead which includes a voltage dropping resistor 49.

The vibratory frequency determining unit for controlling the amplifiersystem described in the preceding paragraphs includes the tuning fork 2having tines 4| and 42 and a field structure upon which the drive andpick-up coils are wound. This field structure comprises a U-shapedpermanent magnet 43 having a pair of legs 44 and 45. A magnetic member46 is electromagnetically connected at one end to the end of the leg 44and a like member 4'! is connected in similar manner to the leg 45.Electromagnetically connected to the other end of the member 46 is athree-legged core structure 48 having outer legs 49 and 50 and a centralleg 5| upon which the drive coil 25 is wound. As shown, the core 48consists of a pair of U-shaped members 52 and 53 mounted side by sidewith their base portions connected to the free end of the member 46 andtheir legs extending inward toward the leg 45 of the magnet 43. Anidentical three-legged core 54 comprising two U-shaped members 55 and 56is electromagnetically connected to the free end of the member 41 withits three legs 5?, 58 and 59 extending toward and in alineinent with thelegs 4-9, 5! and 50 of the core 48. The pick-up coil i5 is wound on thecenter leg 58 of the core 43 in the manner indicated.

In order better to illustrate the details of the fork field structureand to show the flux paths through the magnetic circuit formed by thestructure, reference may be had to the enlarged view of Fig. 2. Asshown, the tines 4| and 42 of the tuning fork 2 are disposed within andbetween the legs of the cores 48 and 54. It will be observed that withthe tines 4i and 42 in unstressed condition between the pole facesformed by the ends of the legs 49, 5|, 59, 5'1, 58, and 55, an air gapis provided between each of the leg ends and a side of an associatedtine. Thus, the pole face formed by the end of the leg 49 is displacedfrom the upper left side of the tine 4! by an air gap A; the end of theleg 5| is displaced from the tines 4| and 42 by gaps B and Crespectively, and the end of leg 50 is displaced from the tine 42 by agap D. In like manner gaps A, B, C, and D are formed between the lowersides of the tines 4| and 42 and the ends of the legs 57, 58 and 59'.The unidirectional flux produced by the permanent magnet 43 is indicatedby dotted lines 4a and it will be seen that this flux traverses themembers 46 and 41, and the gaps A, B, C, D, A, B, C, D and dividesbetween the legs 49, 59 and El and the legs 51, 58, and 59 in accordancewith the relative lengths of the air gaps on either side of the tines 4|and 42. Thus, with the tine 4! in its unstressed position midway betweenthe pole faces formed by the ends of the legs 49 59 and the legs 51 and58, the air gaps B, A, and B are all equal in length and theunidirectional flux will be divided equally between the series-relatedlegs 49 and 5! and the series-releted legs 59 and 59. This, of course,means that in this position of the tines the flux density in the gaps A,A, B and B is the same and the force acting on the tine 4| is zero. Theisolated arrows indicate the direction of the unidirectional flux andshow that the flux traversing the gaps A and B is opposite in directionto the flux traversing the gaps A and B. In like manner. tiunidirectional flux traverses the gaps C and D in a direction oppositeto the direction of the flux in the gaps C and D.

Operation of the above-described apparatus may be initiated byimpressing a current impulse on the drive coil 25. Such an impulseproduces a flux which may be assumed to traverse the paths in the corestructure comprising the legs 49, 59, and 5| in the direction indicatedby the arrows on the ends of the dash lines b. It will be observed thatthis flux traverses the gaps A and B in one direction and the gaps C andD in an opposite direction, The effect of this flux the gaps A and D isto boost the unidirectional flux thereby to increase the flux density inthese gaps. The simultaneous effect of the flux in gaps B and Cresulting from the impulse of starting current is to buck down theunidirectional flux in these gaps thereby to decrease the flux densitytherein. It will be seen that increasing the flux density in gaps A andD while simultaneously decreasing the flux density in gaps B and Cresults in an unbalance of the forces acting on the fork tines 4| and 42in such a direction that the tines expand outward toward the outer legs49 and 50. As the tines expand outward, the reluctance of the gaps B, Band C, C is increased thereby causing the unidirectional flux traversingthe legs 5i and 58 to be decreased and the unidirectional fluxtraversing the legs 49, 50, 51, and 59 to be increased. A decrease inthe flux in the leg 58 causes a feeble impulse of electromotive force tobe generated in the pick-up coil l due to the decrease in the number offiuxlines linking the turns of the coil.

The impulse of electromotive force generated across the terminals of thepick-up coil is impressed on the control grid 1 of the tube l throughthe coupling transformer I 3 and causes an impulse of current to flow inthe output circuit of the tube. A portion of the impulse of energydeveloped in the output circuit is delivered to the drive coil 25through the transformer 23 and causes a current impulse to flow in thewinding 25 in a direction opposite to the direction of current flow ofthe initial current impulse. This produces a flux which traverses thegaps A, B, C and D in directions such that the forces acting on thetines 4| and 42 cause the tines to contract thereby to increase the fluxtraversing the leg 58 and to cause a voltage impulse to be generated inthe pick-up coil I5 having a sign opposite to that caused by theexpansion of the tines. This impulse of voltage exerts a controllingeffect on the tube I such that'the instantaneous current flowing in theoutput circuit thereof is reversed in direction for the duration of theimpulse. The interaction between the fork vibration and amplifier iscumulative, the amplitude of vibration of the fork tines increasinguntil a condition of equilibrium is reached when the fork is vibrated ata substantially constant frequency determined by the amplitude ofalternating current in the drive coil 25, the intensity of theunidirectional flux, and by the natural period of vibration of the fork.After the point of stable operation is reached and during normaloperation of the apparatus, an alternating control voltage is generatedacross the terminals of the pick-up coil i5 by the change in fluxdensity of the flux in the leg 58 caused by the shifting of the fluxbetween the central and outer legs during vibration of the fork tines 4|and 42.

It will of course be understood that the alternating potential developedacross the terminals of the primary winding 22 is impressed on the inputcircuit of the amplifier tube 3 through the coupling condensers 32 and33. This potential controls the operation of the amplifier 3 in a mannerwell understood in the art to cause the same to generate an amplifiedalternating voltage across the output leads 4, 4 having a frequencydetermined by the frequency of vibration of the fork tines 4| and 42.

In order to increase or decrease the rate of vibration of the fork tinesM and 42, a means is provided whereby the intensity of theunidirectional flux may be controlled thereby to control the negativecompliance, or stiffness of the fork tines. This means comprises awinding 60 on the permanent magnet 43 which is connected in series witha battery 6i and a switch 52. If it be'desired to increase' theintensity of the unidirectional flux traversing the air-gaps withinwhich the fork tines vibrate thereby to increase the stiffness of thetines and to decrease the rate of vibration, it is only necessary toclose the switch 62 to boost the magnetization of the permanent magnetto the correct value. Following an increase in the magnetization of themagnet 33, the switch 62 may be opened and the fork tines will continueto vibrate at the newly determined rate. Conversely, if it be desired toincrease the rate of vibration, the polarity of the battery 6! may bereversed and the magnetization of the permanent magnet 63 bucked down toa lower value.

It is desirable to neutralize the effect of voltages generated in thedrive and pick-up coils due to the inductive coupling between the twocoils. Such voltages are reduced to a minimum by loosely coupling thetwo coils and by providing a field structure in which a large leakagereactance is obtained between the individual magnetic circuits of thetwo coils. In accordance with one feature of my invention, effectiveneutralization of the small voltages generated by such inductivecoupling is obtained by the provision of the trans former 2| having itswindings 2t and 26 connected respectively in the pick-up and drive coilcircuits. The windings are connected in a manner such that a potentialis introduced in either of the two circuits which is opposite in phaseto a potential generated by the coupling between the windings I5 and 25and is equal in magnitude to the voltage produced by such coupling. Toillustrate in detail the function of the transformer 2!, let it beassumed that alternating current in the drive coil 25 generates anundesired voltage in the pick-up coil I5. This current traverses thewinding 26 thereby to generate a voltage in winding 20 which is oppositein phase to the voltage generated in the coil l5. By providing thetransformer 25 with the correct turn ratio between windings 28 and 25,the neutralizing voltage across coil 26 will be equal in magnitude tothe voltage in coil i5. In the above manner, effective neutralization isobtained which eliminates stray currents in the pick-up coil circuitthat might adversely affect the controlling actio of the voltagedeveloped across the terminals of the coil.

It is well known that the frequency of the output current of a tuningfork is dependent upon the amplitude of vibration of the fork tines. Itis also known that the amplitude of vibration of the tines is influencedby the magnitude of current supplied to the drive coil of the fork. Itwill be seen, therefore, that it is desirable to provide an arrangementwherein the magnitude of the current in the output circuit of thecontrolled ampliher, from which circuit the current flowing through thedrive coil is derived, be maintained as nearly constant as possible.

It has been found that unavoidable sensitivity changes occur in thecontrol amplifier which-produce changes in the average anode circuitcurrent for a given potential on the control grid of the amplifier. Inother words, the plate currentgrid voltage characteristic curve isdisplaced each time the sensitivity of the amplifier changes. Suchsensitivity changes may result from a number of causes such asfluctuations in the voltage of the cathode and anode energy supplysource and changes in the tube characteristics. It is desirable toprovide a control amplifier in which such changes in sensitivity producethe smallest obtainable change in current amplitude in the outputcircuit for, as is noted above, such changes produce a change in theamplitude of vibration of the fork tines and an attendant change in theoutput frequency of the oscillator. If an amplifier be employed havingthe above operating characteristics, the frequency shift for a givenamplifier sensitivity change is considerably minimized.

In accordance with my invention, the abovenoted desired operatingfeatures are obtained by providing a circuit for supplying a biasingpotential to the control electrode 1 of the tube i which varies inaccordance with the intensity of current flowing in the output circuitconnected between the anode 6 and the cathode 5. This circuit functionsto maintain substantially constant the anode current and, in addition,operates to insure a linear relation between changes in the amplitude offork vibration and variations in the sensitivity of the amplifier. Theautomatic biasing circuit comprises a diode formed by the element I andthe cathode of the tube I, a coupling condenser 63, and resistances 64and 65.

The above-described circuit operates to exert a controlling influence onthe amplifier in the following manner: During normal operation of theoscillator the alternating potential existing between the cathode andanode of the tube I is impressed across the element III and the cathode5 by means of the coupling condenser 63. A unidirectional current flowsbetween the two elements noted, thereby to develop a unidirectionalelectromotive force which is impressed on the resistance 64 connectedacross the tube elements 5 and It. The potential developed across theresistance 64 is impressed on the control electrode 1 through theresistance 65 and the secondary I2 of the coupling transformer I3. Itwill be seen that the potential drop across the diode of the tube Icorresponds to the drop across the terminals of the anode circuit and,accordingly, is proportional to the intensity of current flowing in theanode circuit. This, of course, means that the biasing potentialimpressed on the control grid I of the tube I by means of the resistance64 varies with, and in proportion to, the magnitude of current flowingin the anode circuit of the amplifier. The controlling effect of thisbiasing potential is such that as the anode circuit current starts toincrease a negative bias is impressed on the control grid I which tendsto reduce the anode current. Conversely, if the value of the anodecircuit current starts to decrease, the negative bias is reduced,thereby to permit an increased current fiow in the anode circuit of theapparatus. In this manner, variations in the magnitude of current in thecircuit are minimized which, of course, means that fluctuations in thecurrent supplied to the drive coil 25 are lessened. Although I havedescribed the automatic biasing circuit in its preferred form asincluding a diode formed within the tube I, it will of course beunderstood that an additional tube may be employed and that variousmodifications of the circuit may be made which will operate in asatisfactory manner.

The desirability of using a thermionic tube I having a screen griddesigned to prevent interelectrode capacity coupling between the inputand output circuits connected to the tube has already been mentioned.Screen grid tubes are ordinarily rendered operative to prevent suchundesired capacity coupling by providing a circuit for maintaining theauxiliary grid at a positive potential slightly below that of the anodeof the tube. In the circuit arrangement illustrated, this positivepotential is supplied to the grid 8 through the circuit lead whichincludes the voltage dropping resistance 4%. It is well known thatfluctuations in the magnitude of the positive potential applied to theauxiliary grid produce corresponding fluctuations in the amplitude ofcurrent flowing in the anode circuit of the tube.

In accordance with my invention, such fluctuations are minimized by theprovision of an electric discharge device 55 which is connected betweenthe screen grid 8 and the cathode 5 and in series with the resistance 48across the source of potential The device 65 may be an ordinary glowdischarge tube of which there are several wellknown commercial examplesin the art. Briefly,

the tube comprises a pair of spaced apart electrodes positioned within acontainer filled with an ionizable medium such as neon.

The tube 66 possesses an operating characteristic such that a highervoltage is required to start a glow discharge within the tube than isnecessary to maintain the discharge after the tube has once beenstarted. In order to supply a high voltage impulse for starting thetube, a switch 61 is provided having its contacts connected to theterminals of the circuit. When the switch 61 is quickly closed andopened a voltage impulse is developed which is of sufiicient magnitudeto start a discharge in the tube 66. The impulse is caused by the slightreactance of the impedance element 40 and the reactance of the circuitleads.

When the tube 66 is started, it operates to draw a current which isproportional to the voltage of the source 21. Thus, as the voltageincreases the current fiowing through the resistance 40 and the tube 66increases. This increase in current increases the voltage drop acrossthe resistance 40 thereby to maintain the junction point between thescreen grid circuit lead, the terminal of tube 66 and the terminal ofresistance 40 at the same potential with respect to the oaths ode 5 asbefore the change occurred. Conversely, if the voltage of the source 21decreases, the current through the tube 66 decreases and the potentialof the junction point remains unchanged. Thus, by providing thedischarge device 66 connected in the manner illustrated fluctuations inthe anode current flowing in the output circuit of the amplifiers areminimized, and the magnitude of current supplied to the drive coil 25 ismaintained substantially constant.

The desirability of providing the automatic control grid biasing circuitand the voltage regulator 66 in the manner set forth in the immediatelypreceding paragraphs is emphasized and rendered more clearlyunderstandable by the curves shown in Fig. 3 which illustrate certain ofthe operating characteristics of my improved oscillation generator. Inthis figure, the curves E and E indicate the current flowing in thedrive coil 25 as a function of the grid potential on the control grid Iof the discharge tube I, and the straight line F illustrates theamplitude of vibration of the fork tines 4| and 42 as a function of thecurrent flowing in the drive coil 25. It will be seen that the curve Eis substantially fiat along its upper right-hand portion in the regionwhere it intersects the curve F, as at point G. It will be understoodthat if the current in the anode circuit of the tube I be maintainedsubstantially constant so that the characteristic curve E is not shiftedfrom the position shown, the point of intersection G between the curvesE and F will remain fixed and the amplitude of vibration of the forktines will remain absolutely constant. The provision of the voltageregulator 66 and the circuit for supplying a control potential to thecontrol grid I which varies in accordance with the intensity of currentin the anode circuit of the amplifier I tends to maintain this desiredstability of the operating point G. However, if for some uncontrollablecause, the sensitivity of the amplifier changes in such a way that theamplifier characteristic curve E is shifted to the position shown indotted lines at E, the point of intersection G is shifted downwardlyalong the curve F until the point of stable operation G is reached.However, due to the flattopped. characteristic of the curves E and E,the change H, H in the amplitude of fork tine vibratines.

tion caused by such a shift of the operating point G is substantiallydirectly proportional to the Change in sensitivity of the amplifier.This, of course, means that linear relation exists between a particularvariation in the sensitivity of the amplifier and an attendant variationin the amplitude of vibration of the fork tines.

The importance of employing an amplifier having a characteristic curve Ewith a fiat top will be more readily appreciated by reference to Fig. 4wherein I have shown characteristic curves corresponding to those shownin Fig. 3 for an amplifier typical of those used in connection withoscillation generators now known to the art. In Fig. 4, like referencecharacters are used to denote characteristic curves corresponding tothose of Fig. 3. It will be seen that the curve E is substantiallystraight throughout its length and that it possesses a slope in theregion of point G corresponding to the point of intersection between thecurves E and F which closely approaches the slope of the curve F. Thismeans that if the sensitivity of the amplifier changes to shift thecharacteristic E to a new position E, the operating point G will beshifted a considerable distance along the curve F, thereby producing achange H, H in the amplitude of fork tine vibration which isdisproportionately large for the decrease in amplifier sensitivity. Bycomparing the curves of Fig. 3 with those of Fig. 4, it will be seenthat for a given amplifier sensitivity change, the change in amplitudeof fork tine vibration is substantially less for my improved apparatus.Thisv of course means that the change in the output frequency resultingfrom such an amplifier sensitivity change is less.

A further advantage attendant upon the use of my improved circuit willbe apparent from a consideration of the function of the apparatus duringthe starting period thereof. Thus, referring again to the curves of Fig.3, and considering the operation of the apparatus during the startingperiod, if an impulse of current be generated in the output circuit ofthe amplifier having a value I, for example, the anode current necessaryto produce sustained vibration of the fork tines at an amplitude OJ isJK. The differential of current IK is of considerable magnitude andtends to increase the amplitude of vibration of the However, an increasein the amplitude of vibration produces a coincident increase in theanode current. This differential of current IK produces a cumulativeincrease in the amplitude of fork tine vibration, until the point ofintersection G between the curves E and Fis reached when the fork tinescontinue to vibrate with an amplitude OH. The feature of importance tothe improved operation of the apparatus is the maintenance of aconsiderable differential between the anode current flowing in theamplifier output circuit and that necessary to produce an amplitude ofvibration corresponding to the current developed. This insures a quickand positive starting of the apparatus and prevents drift ing of theoperating point G during normal op eration of the oscillator.

Considering the curves of Fig. 4 by way of contrast, it will be observedthat the curves E and F closely parallel each other and at no timeduring the starting period when the amplitude of fork tine vibration isbuilding up along curve F is there a substantial dilference between theanode current developed and the current necessary to produce stablevibration at the amplitude of vibration prevailing at any given instantduring the starting period. It will further be seen that the two curvesintersect at a very small angle. The small current differential betweenthe curves E and F means that the apparatus is sluggish in acceleratingto a normal operating condition. The very small angle of intersectionbetween the two curves means poor frequency regulation resulting from adrift in the amplitude of fork tine vibration after the normal operatingpoint G is reached.

The desirability of maintaining the intensity of the unidirectional fluxproduced by the magnet 43 has been previously mentioned. Thisrequirement for satisfactory operation of the apparatus is based on thefact that the intensity of the unidirectional fiux determines thestiffness, or negative compliance, of the fork tines which are traversedby this flux. It will be seen that, if during the operation of theapparatus the flux density changes the stiffness of the fork tines willbe altered thereby to produce a change in the natural period ofvibration of the fork which is accompanied by a change in the outputfrequency of the generator. It is imperative, therefore, that theintensity of the unidirectional flux be maintained substantiallyconstant at all times during the operation of the oscillation generator.

Referring to Figs. 5 and 6 of the drawings, I have shown side viewspartially in section illustrating a physical embodiment of my inventionin which means are provided for opposing changes in the intensity of theunidirectional fiux traversing the field structure of the vibratingsystem. As shown, the field structure assembly and the transformer 2!are mounted on a supporting plate 68. This plate may form the cover fora thermally insulated container (not shown) within which the tuning forkis mounted. Preferably, the temperature of the fork should be maintainedconstant to prevent variations in the physical characteristics thereof.An even temperature may be obtained in accordance with the usualprocedure by providing thermostatically controlled heating means withinthe surrounding container.

In order to minimize variations in the intensity of the unidirectionalflux produced by the permanent magnet 43, a pair of closed circuitedconductors 69 and 1B are provided which surround respectively the fieldpieces 46 and 4'! forming a part of the magnetic circuit of the fieldstructure. The closed circuited conductors 69 and 70 are constructed ofnonmagnetic material and, preferably, are formed from small copperblocks provided with openings of sufficient size to 'accommodate thefield pieces 46 and 41. The turns 69 and 10 function to oppose transientchanges in the intensity of the unidirectional fiux thereby to maintainthe flux substantially constant in the desired manner.

This is important since it maintains a constant flux through the magnetnotwithstanding vibration of the fork. For example, if we assume anordinary vibrating reed oscillating in front of a pair of pole pieces ofa magnet it will be observed that the flux in the magnet must vary inaccordance with the position of the reed. This is undesirable not onlybecause such changes are repugnant to the natural character of themagnet to maintain constant its magnetism, but in addition, equaldeflections of the reed in opposite directions produce unequalvariations in the fiux in the magnet. Thus if a coil be wound on themagnet the electromotive force induced therein is not sinusoidal but isdistorted, the amplitude Gil during positive half cycles being fargreater than during negative half cycles. In the device described theunidirectional flux is constant and is maintained so by the arrangementof the fork with respect to the pole pieces as shown and by the heavyconducting rings 69 and 79. Thus the forces applied to the vibratoryelement and the electromotive force induced in the pick-up coil are morenearly sinusoidal, the undesired harmonic components which otherwiseoccur being substantially reduced.

In order to permit minor adjustments of the apparatus to be made duringthe assembly and installation thereof, the tuning fork field structureincluding the permanent magnet 43 is preferably assembled to form amovable unit in the manner clearly illustrated in the perspective Viewof Fig. '7 of the drawings. As shown, the field structure is mountedwithin a rectangular supporting frame comprising a pair of end-pieces Hand a pair of side pieces 12. The supporting frame is in turn mounted onthe plate 68 by stud screws 13 which extend through openings in theend-pieces II and are threaded into the cover plate 68. The holes in theend-pieces II should be of a diameter slightly larger than the diameterof the stud screws 73, thereby to permit the field unit and thesupporting frame to be moved about on the surface of the plate 68. Thispermits a ready positioning of the field structure to secure the correctlengths of the air gaps A, B, C, D and A, B, C, D after the vibratingsystem has been assembled.

From the foregoing description of my improved oscillation generatingapparatus, it will be apparent that I have devised an apparatus which isquick starting, which is dependable in operation, and which is equippedto produce at its output terminals an alternating control voltage havinga very stable frequency characteristic under all operating conditions.In a specific clock system having my improved oscillation generatorembodied therein, the stability and frequency of the generator was suchthat the deviation in time from the correct time was less than one-tenthof a second over a twenty-four hour period. In addition, my improvedapparatus is of simple and economical structure and may be constructedwith sufficient mechanical rigidity to withstand hard usage under themost adverse operating conditions.

While I have shown a particular embodiment of my invention, it will ofcourse be understood that I do not wish to be limited thereto since manymodifications in the circuit arrangement and the structure may be made,and I contemplate by the appended claims to cover all such modificationsas fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An oscillation generator comprising a vibratory element, an electrondischarge device including an anode, a cathode and a control grid, inputand output circuits connected to said device, means including a drivecoil coupled to said o put circuit for vibrating said element, meanshiding a pick-up coil for controlling the frequency of current in saidoutput circuit in accordance with the frequency of vibration of saidelement, and means for maintaining the amplitude of vibration of saidelement substantially con and, said last-named means including a circuitfor impressing a control potential of said control grid which varies inaccordance with the each pole having a pair intensity of current in saidoutput circuit and in such a direction as to reduce the amplification ofsaid electron discharge device when said intensity of output currentincreases.

2. An oscillation generator comprising a vibratory element, a drive coilfor vibrating said element, an electron discharge amplifier connected toenergize said drive coil during operation of said generator, saidamplifier including an output circuit and a control electrode, a pick-upcoil connected to control the frequency of current alternations in saidoutput circuit in accordance with the frequency of vibration of saidelement, and means for rapidly increasing during the starting period ofthe generator the amplitude of vibration of said element, said lastnamed means including a circuit for impressing on said control electrodea control potential which varies in accordance with the intensity ofcurrent in said output circuit in a direction to reduce theamplification of said amplifier when said intensity of output currentincreases.

3. An oscillation generator comprising a vibratory element, a drive coilfor vibrating said element, an amplifier including an electron dischargedevice having an anode, a cathode and a control grid, input and outputcircuits connected to said device, a source of anode potential connectedin said output circuit, means for energizing said drive coil inaccordance with the alternating current in said output circuit therebyto vibrate said element, means including a pickup coil coupled to saidinput circuit for controlling the frequency of current alternations insaid output circuit. and means for maintaining a substantially inverselinear relation between variations in the sensitivity of said amplifierand variations in the amplitude of vibration of said element, said lastnamed means including a circuit for supplying a control potential tosaid control grid which varies in accordance with the amplitude ofcurrent in said output circuit.

4. An oscillation generator including a vibratory element, amplifier,means including a drive coil coupled to said amplifier for vibratingsaid element, and means for generating a control potential for saidamplifier having a frequency determined by the frequency of vibration ofsaid element, said last-named means including a magnetic core having apair of poles, each pole having a pair of legs, said element beingpositioned between the legs of both pairs to provide an air gap betweeneach of said legs and said element, said drive coil being mounted on aleg of one of said pairs and said pick-up coil being mounted on a leg ofthe other of said pairs, and means for producing a unidirectional fluxwhich traverses said legs and extends from one of said poles to theother whereby said unidirectional flux shifts alternately from one legof each pair to the other during vibration of said element therebyalternately to increase and decrease the fiux linking said pick-up coiland generate said control potential.

5. An oscillation generator including in combination, a magnetic corehaving a pair of poles, of legs, a pick-up coil on one of said legs ofone pole, a vibratory element disposed between said legs of both polesto provide an air gap between each of said legs and said element, meansfor energizing said core to produce a unidirectional flux whichtraverses said legs in the same direction and which shifts alternatelyfrom one leg of each pair to the other during vibration of said elementthereby to generate an alternating voltage in said coil having afrequency determined by the frequency of vibration of said element, andmeans for driving said element thereby to vibrate said element at itsnatural frequency.

6. An oscillation generator including in combination, an amplifierhaving input and output circuits, a vibratory element, a magnetic corehaving a pair of poles, each pole having a pair of legs, a pick-up coilon one of said legs of one pole, means for coupling said pick-up coil tosaid input circuit, said element being disposed between said legs ofboth poles to provide an air gap between each of said legs and saidelement, means for energizing said core to produce a unidirectional fiuxwhich traverses said legs in the same direction and which shiftsalternately from one leg of each pair to the other during vibration ofsaid element thereby to generate an alternating voltage in said pick-upcoil having a frequency determined by the frequency of vibration of saidelement, and means including a drive coil coupled to said output circuitfor driving said ele- V to vibrate at its natural frequency.

'7. An oscillation generator including in combination, a magnetic corehaving a pair of poles, each pole having a pair of legs, a pick-up coilon one of said legs of one pole, a vibratory element disposed betweensaid legs of both poles to provide an air gap between each of said legsand said element, means for energizing said core to produce aunidirectional flux which traverses said legs in the same direction andwhich shifts alternateiy from one leg of each pole to the other duringvibration of said element thereby to generate an alternating voltage insaid coil having a frequency determined by the frequency of vibration ofsaid element, means for opposing instantaneous changes in the intensityof said unidirectional fiux and means for driving said element therebyto vibrate said element at its natural frequency.

8. An oscillation generator comprising in combination, an amplifierhaving input and output circuits, a magnetic structure including two U-shaped cores having their legs extending toward each other, a pick-upcoil on one of the legs of one of said cores, means for coupling saidpick-up coil to said input circuit, a drive coil on one of the legs ofthe second of said cores,

means for coupling said drive coil to said output circuit, saidvibratory element being disposed within said legs to provide an air gapbetween each of said legs and said element, means for energizing saidcore to produce a unidirectional flux which traverses said legs in thesame direction and which shifts between the legs of said one core duringvibration of said element thereby to generate an alternating voltage insaid i pick-up coil having a frequency determined by the frequency ofvibration of said element, said legs of said second core beingpositioned on either side of said element whereby said unidirectionalflux traverses the air gaps between said legs and said element inopposite directions, and means including said amplifier for energizingsaid drive coil to produce an alternating flux which instantaneouslytraverses said last named gaps in the same direction whereby the fluxdensity in said gapsiis alternately increased and decreased inaccordance with said alternating flux thereby to produce sustainedvibration of said element.

9. An oscillation generator comprising in combination, a vibratoryelement, a drive coil for vibrating said element, an amplifier includingan electron discharge device having an anode, a cathode, a control gridand a screen grid, input and output circuits connected to said device, asource of anode potential connected in said anode circuit, means forsupplying current to said drive coil having a frequency determined bythe frequency of the current in said output circuit thereby to vibratesaid element, means including a pick-up coil coupled to said inputcircuit for controlling the frequency of the current in said outputcircuit, and means for maintaining the intensity of current in saidoutput circuit substantially constant thereby to maintain the frequencyof vibration of said element substantially constant, said last-namedmeans including a circuit connected to said screen grid and having asource of potential connected therein, and means connected in saidcircuit for maintaining substantially constant the potential impressedon said screen grid.

10. In combination, a tuning fork having tines disposed between a pairof pole pieces, each of said pole pieces having three legs, one leg ofeach pole piece extending between the tines of said fork and the otherlegs of each pole piece straddling said fork, means to produce aconstant magnetic field between said pole pieces and a winding on oneleg of each pole piece.

11. In combination, a mechanically vibratory device having an excitingwinding and a second winding, an amplifier having an anode, a cathodeand a control electrode coupling said second winding and said excitingwinding, a source of anode potential and a source of cathode heatingcurrent for said amplifier, the amplitude of Vibration of said vibratorydevice being proportional to the magnitude of alternating electromotiveforce supplied to said exciting winding, means including said amplifierto transmit electromotive force from said second winding to saidexciting Winding, and means operating independently of variations insaid sources to cause the electromotive force supplied to said excitingwinding to increase at a progressively diminishing rate with respect tothe electromotive force in said second winding during the startingperiod of said vibratory device, said last named means including acircuit for impressing a potential on said control electrode whichvaries inversely in accordance with the intensity of current in theanode-cathode circuit of said amplifier.

12. In combination, a mechanically vibratory device having an excitingwinding and a second winding, an amplifier including an output, circuitand a control electrode coupling said second winding and said excitingwinding to transmit the electromotive force induced in said secondwinding to said exciting winding in amplified form, the amplification ofsaid amplifier when said electromotive force is small being sufiicientlygreat to cause rapid acceleration of said vibratory device intovibration, and means to reduce said amplification when said vibratingdevice has attained substantially its normal ampli tude of vibration toa value suficient to maintain said normal amplitude of vibration, saidmeans including a circuit for impressing a potential on said controlelectrode which varies inversely in accordance with the intensity ofcur- ,rent in said output circuit.

13. An oscillation generator comprising in combination, a'magnetic corehaving a pair of opposed main poles, each pole including a pair ofopposed pole faces, a vibratory element hav ing portions thereofdisposed between the pole faces of both said pairs of opposed pole facesto provide an air gap between each of said pole faces and said element,said element being arranged to vibrate in the plane of said main polesand in such direction as to vary the length of said air gaps, means forenergizing said magnetic core to produce unidirectional flux whichtraverses the gaps between each pair of oppositely disposed pole facesand said element in opposite directions, and means for energizing saidmagnetic core to produce an alternating flux which instantaneouslytraverses the last mentioned gaps in the same direction whereby the fluxdensity in said last mentioned gaps is alternately increased anddecreased in accordance with said alternating flux thereby to producesustained vibration of said element.

14. An oscillation generator comprising in combination, a magnetic corehaving a. pair of opposed main poles, each pole including a pair ofopposed pole faces, a vibratory element having portions thereof disposedbetween the pole faces of both said pairs of opposed pole faces toprovide an air gap between each of said pole faces and said element,said element being ar ranged to vibrate in the plane of said main polesand in such direction as to vary the length of said air gaps, means forenergizing said magnetic core to produce unidirectional flux whichtraverses the gaps between each pair of oppositely disposed pole facesand said element in opposite directions, means for energizing saidmagnetic core to produce an alternating flux which instantaneouslytraverses the last men tioned gaps in the same direction whereby theflux density in said last mentioned gaps is alternately increased anddecreased in accordance with said alternating flux thereby to producesustained vibration of said element, and means for controlling thefrequency of said alternating flux in accordance with the frequency ofvibration of said element.

MILTON S. MEAD, JR.

